MFV r338866: 9700 ZFS resilvered mirror does not balance reads

illumos/illumos-gate@82f63c3c2bf5e4378706e8dcfccf717d67371be9

Reviewed by: Toomas Soome <tsoome@me.com>
Reviewed by: Sanjay Nadkarni <sanjay.nadkarni@nexenta.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Matthew Ahrens <mahrens@delphix.com>
Author: Jerry Jelinek <jerry.jelinek@joyent.com>

Approved by: re (delphij)